The present invention relates to a descrambling apparatus which is used for a CATV (community antenna television or cable television).
At present, as a new broadcasting service, a television broadcast using cables as a transmitting medium (CATV) is widespread. In this type of broadcast, the service is provided to only specific contractors (i.e. CATV subscribers); therefore, a particular modulation (this is called a scramble) is performed on the television signal to be transmitted at the side of the transmitting station of the television signal so that the non-contractors cannot receive this broadcast, thereby preventing reception of the television signal by the non-contractors. Each contractor has a descramble apparatus for descrambling the scrambled waveforms, so that he can receive the CATV service. The present invention relates to such a descrambling apparatus.
FIG. 1 is a schematic diagram illustrating the waveforms of the ordinary television signal which is not scrambled, in which the transverse axis, indicates time and the vertical axis represents the signal amplitude level.
The waveforms shown in FIG. 1 are the waveforms to which amplitude modulation was performed using the RF signal as a carrier wave, and a video signal consists of a video information signal section and a blanking signal section. A synchronizing signal S is added to the blanking signal section.
When the receiver receives such a television signal, it detects the envelope of those waveforms, thereby enabling the sync signal S whose level is higher than the level of the blanking signal section to be easily extracted. In this way, the correct reception of the video signal can be performed.
In case of CATV, if the television signal having such waveforms as shown in FIG. 1 is transmitted as it is from the broadcasting station, anybody as well as non-contractors can receive it; therefore, this method disadvantageously departs from the inherent requirement of CATV that the services are provided to only the contractors. To solve such a problem as mentioned above, the broadcasting station of the CATV transmits the television signal having particular waveforms which were scrambled in such a manner that ordinary receivers cannot easily separate the sync signal S and, accordingly, they cannot receive the accurate video signal, and each contractor has a special apparatus for descrambling such scrambled waveforms, thereby enabling the correct reception of the television pictures.
FIG. 2 is a schematic diagram showing the scrambled waveforms of a television signal to be used in a conventional scrambling/descrambling method, which is called a pilot carrier method. Such a method has been employed in the products by Hamlin International Corp., such as CATV converters MODEL MCC-4000-P, MCC-3000-P, SPC-4000-P and SPC-3000-P (1978).
In contrast to the waveforms shown in FIG. 1, the waveforms shown in FIG. 2 are the waveforms having a pilot signal PS which has a special frequency spectrum different from that of the carrier waves (this is referred to as a video carrier) on which the video signal is added and whose amplitude level is slightly lower than that of the blanking signal, and this pilot signal PS is interposed as the sync signal S between the blanking and video information signal sections.
One or more number of, or any number of pilot signals may be used as the sync signal. In this type of scrambled waveform, which is different from the standard waveform of FIG. 1, since the frequency band of the pilot signal PS is different from that of the video carrier by using for example another channel, the necessary frequency band becomes wide, so that the frequency band which is about twice that of one channel as required in the case of the waveforms of FIG. 1 is needed as the band for one channel.
FIG. 3 is a circuit diagram showing a descrambling apparatus for the scrambled waveforms according to the pilot carrier method shown in FIG. 2. In the diagram, a reference numeral 1 denotes a band pass filter (BPF) to pass only the video carrier frequency band; 2 is a primary mixer circuit; 3 is a primary local oscillator; 4 is an IF band pass filter; 5 is an amplifier; 6 is a secondary mixer circuit; 7 is a secondary local oscillator; 8 is a modulator; 9 is a BPF to pass only the frequency band of the pilot signal; 10 is an amplifier; 11 is a detector; and 12 is a Schmitt trigger circuit (which has a certain threshold value and when the input signal level exceeds this threshold value level, it outputs an output signal). The upper route consisting of the circuit blocks 1 to 8 indicates the ordinary route for processing the received television signal, while the lower route consisting of the circuit blocks 9 to 12 represents the route for extracting the sync signal S (i.e. pilot signal PS) from the received television signal.
The operation of the circuit of FIG. 2 will now be described. Such scrambled waveforms as shown in FIG. 2 (however, in this case, it is assumed that only one pilot signal PS is shown) are input to an input terminal IN. Since the BPF 1 passes only the video carrier frequency band (RF frequency), the pilot signal PS has been eliminated from the output of the filter 1. This output (the blanking and video information signals on the video carrier) of the BPF 1 is frequency converted by the primary mixer circuit 2 and local oscillator 3 and the beat-down is performed to obtain the IF frequency. Then, the signal passes through the IF band pass filter 4, where the levels of the video information and luminance signals (and further the audio signal) are adjusted. After they have been amplified by the amplifier 5, the output is again frequency converted into the RF frequency by the secondary mixer circuit 6 and local oscillator 7, and is input as a modulated signal to the modulator 8.
On the other hand, the pilot signal PS (sync signal) is extracted from the scrambled waveforms input to the input terminal IN by the BPF 9, this pilot signal PS is amplified by the amplifier 10, thereafter it is detected by the detector 11. After the level of the detection output has been set into a proper level by the Schmitt trigger circuit 12, it is input as a modulation signal to the modulator 8. In the modulator 8, the modulation is executed to a the blanking signal (and the video information signal) on the video carrier of the RF frequency as waves to be modulated using the detection output of the pilot signal PS as modulation waves, thereby outputting such waveforms thus descrambled as shown in FIG. 1 from an output terminal OUT.
The above is the description of conventional scrambled waveforms and the descrambling apparatus to descramble those waveforms. However, as already described, with such a conventional pilot carrier scrambling method, there is a drawback in that it is difficult to realize the multichannel in the broadcasting channel since the necessary frequency band becomes wide.